A large number of adjusting devices are already known in which the flow rate of a fluid is controlled by means of a movable element, the position of which determines the cross-section of flow in the device.
These known devices do not however allow precise adjustment, particularly over a long period, as they do not take the actual position of the movable element into account and/or do not allow for the physical changes which affect the mechanical parts of the devices.
In the latter case wear on the packing of the movable element and/or the contact surface of that element will cause the fully closed position of the adjusting device to be shifted, and this will not be taken into account.
Moreover ageing of the materials of the mechanical elements, possible deformation and play of the motion transfer members and possible changes of position between the actuators and the position sensors might cause additional adjustment errors.
These adjustment faults prevent accurate adjustment of the flow even if a high-precision actuator is used. They are particularly prejudicial to devices providing proportional control, where the closed position given by the sensor no longer corresponds to an actual state of total sealing.
Furthermore the majority of known adjusting devices do not check or monitor their state, although such action would probably avert the risks caused by a major fault or malfunctioning and ensure that the devices could be used under normal predetermined conditions.